FIG. 1 is a schematic diagram of the structure of a coherent optical communication system. As show in FIG. 1, in the coherent optical communication system, optical signals are transmitted from the transmitting end to the receiving end via optical channels. Dispersion in the optical fiber and polarization mode dispersion will induce inter-symbol interference (ISI). In order to compensate for the ISI induced by the optical channels, equalization is generally used in the receiver. The equalization may be divided into two steps: the first step is static equalization for compensating for the damages that are induced by the optical channels and do not vary along with the time (such as accumulated dispersion), and the second step is adaptive equalization (AEQ) for compensating for the residual dispersion and the damages varying along with the time (such as polarization scattering and polarization mode dispersion). An adaptive equalizer is generally realized by a finite impulse response (FIR) filter. In a dual-polarization system, a butterfly FIR filter is used as an equalizer.
FIG. 2 is a schematic diagram of the structure of a butterfly FIR filter. As show in FIG. 2, the FIR tap coefficients are denoted by vectors wxx, wyx, wxy and wyy, and the relations between the input and output of the equalizer are as shown by formulae (1x) and (1y):sx=wxxrx+wyxry  (1x)sy=wxyrx+wyyry  (1y);
where,  denotes a convolution operation, r is an input signal of the equalizer, s is an output signal of the equalizer, and the two subscripts x, y denote two polarization modes.
Currently, there are mainly two conventional adaptive equalization methods: a constant modulus algorithm (CMA) and a decision-assisted minimal mean square error (MMSE) algorithm, in both of which an initial tap coefficient (weight) is set first for the FIR filter, and then the tap coefficient is iteratively updated according to a certain cost function, making it converged to an optimal value. However, any information on related channels is not used in setting an initial tap coefficient for the FIR filter, which may be much different from the optimal value, and need a relatively long converged procedure in reaching the optimal value.
It should be noted that the above description of the background art is merely provided for clear and complete explanation of the present invention and for easy understanding by those skilled in the art. And it should not be understood that the above technical solution is known to those skilled in the art as it is described in the background art of the present invention.